Bioinspired light-driven soft robots based on liquid crystal polymers.

Nature is a constant source of inspiration for materials scientists, fueling the dream of mimicking life-like motion and tasks in untethered, man-made devices. Liquid crystalline polymers (LCPs) programmed to undergo three-dimensional shape changes in response to light are promising materials for fulfilling this dream. The successful development of autonomous, highly controlled light-driven soft robots calls for an understanding of light-driven actuation, advancements in material function and performance, and progress in engineering principles for transforming actuation into life-like motions, from simple bending to walking, for example. This tutorial review includes an introduction to liquid crystal (LC)-based materials and highlights developments in light-responsive LC polymers, shape programmability and sustained motions to finally achieve bioinspired untethered soft robots able to perform locomotion and tasks.

Fluorene benzothiadiazole co-oligomer based aqueous self-assembled nanoparticles.

Self-assembled π-conjugated nanoparticles with tunable optical characteristics are appealing for sensing and imaging applications due to their intrinsic fluorescence, supramolecular organization and dynamics. Here we report on the facile synthesis of fluorene benzothiadiazole co-oligomers in which structural backbone alterations induce bathochromically shifted optical characteristics. Moreover, the nature of the oligomer side-chains revealed the role of bulkiness and polarity on the optical and self-assembly behavior. Co-assemblies were prepared that showed energy transfer between the different oligomers which allows for tuning of the emission color. These compounds thus extend the π-conjugated-oligomer toolbox from which nanoparticles can be prepared with tailored physicochemical properties that may result in supramolecular materials for biosensing.

Patterned oscillating topographical changes in photoresponsive polymer coatings.

The light-induced surface topography of a liquid crystal polymer coating is brought into a patterned oscillatory deformation. A dichroic photo-responsive azobenzene is co-aligned with the planar oriented nematic liquid crystal network molecules which makes the surface deformation sensitive to polarized UV light. Locally selective actuation is achieved in coatings with a complex alignment pattern. Dynamic oscillation, as controlled by the actuation and relaxation kinetics of the polymer, is obtained by a continuous change in the polarization of the UV source. The atypical deformation at the defect lines between the domains is of special interest. The amplitude and presence of the oscillation can be manipulated by changing the ratio between blue and UV light and by varying the ambient temperature of the coating.

Reactive oligo(dimethylsiloxane) mesogens and their nanostructured thin films.

Oligo(dimethylsiloxane)-based reactive mesogens were prepared and shown to form room-temperature smectic phases which were 'frozen-in' by photopolymerization. Homeotropically aligned, nanostructured thin films were obtained by spincoating, and micropatterning was demonstrated. These hybrid reactive mesogens are suitable for the preparation of aligned nanostructured polymer thin films with potential applications ranging from stimuli responsive coatings to nanoporous membranes.

Energy transfer processes along a supramolecular chain of π-conjugated molecules.

We have investigated the energy transfer dynamics in a supramolecular linear polymer chain comprising oligofluorene (OF) energy donor units linked by quadruple hydrogen-bonding groups, and oligophenylene (OPV) chain ends that act as energy acceptors. Using femtosecond spectroscopy, we followed the dynamics of energy transfer from the main chain of OF units to the OPV chain ends and simulated these data taking a Monte Carlo approach that included different extents of electronic wave function delocalization for the energy donor and acceptor. Best correlations between experimental and theoretical results were obtained for the assumption of electronic coupling occurring between a localized donor dipole moment and a delocalized acceptor moment. These findings emphasize that geometric relaxation following initial excitation of the donor needs to be taken into account, as it leads to a localization of the donor's excited state wave function prior to energy transfer. In addition, our simulations show that the energy transfer from the main chain to the ends is dominated by an interplay between slow and spatially limited exciton migration along the OF segments comprising the main chain and the comparatively faster hetero-transfer to the end-cap acceptors from directly adjoining OF segments. These results clearly support the description of host-guest energy transfer in linear polymer chains as a two-step mechanism with exciton diffusion in the host being a prerequisite to energy transfer to the guest.

Anharmonic magnetic deformation of self-assembled molecular nanocapsules.

High magnetic fields were used to deform spherical nanocapsules, self-assembled from bolaamphiphilic sexithiophene molecules. At low fields the deformation--measured through linear birefringence-scales quadratically with the capsule radius and with the magnetic field strength. These data confirm a long standing theoretical prediction [W. Helfrich, Phys. Lett. A 43, 409 (1973)10.1016/0375-9601(73)90396-4], and permit the determination of the bending rigidity of the capsules as (2.6+/-0.8) x 10(-21) J. At high fields, an enhanced rigidity is found which cannot be explained within the Helfrich model. We propose a complete form of the free energy functional that accounts for this behavior, and allows discussion of the formation and stability of nanocapsules in solution.

Electrical transport measurements on self-assembled organic molecular wires.

The electrical properties of supermolecular assemblies of oligo(p-phenylene vinylene) were studied. These materials self-assemble into well-defined cylindrical structures in solution with lengths in the range of 100 nm-10 microm and diameters between 5 and 200 nm. Atomic force microscopy showed that by adjusting the concentration, either individual molecular wires or a dense film could be deposited. The molecular wires showed poor electrical conduction. Several tests were performed that show that it was the molecular wires themselves, not the contacts, that limit the conductivity.

Excitation migration along oligophenylenevinylene-based chiral stacks: delocalization effects on transport dynamics.

Atomistic models based on quantum-chemical calculations are combined with time-resolved spectroscopic investigations to explore the migration of electronic excitations along oligophenylenevinylene-based chiral stacks. It is found that the usual Pauli master equation (PME) approach relying on uncoherent transport between individual chromophores underestimates the excitation diffusion dynamics, monitored here by the time decay of the transient polarization anisotropy. A better agreement to experiment is achieved when accounting for excitation delocalization among acceptor molecules, as implemented in a modified version of the PME model. The same models are applied to study light harvesting and trapping in guest-host systems built from oligomers of different lengths.

Highly emissive supramolecular oligo(p-phenylene vinylene) dendrimers.

pi-Conjugated oligo(p-phenylene vinylene) (OPV) guest molecules for interaction with dendritic hosts were synthesized and fully characterized by NMR spectroscopy, MALDI-TOF-MS, elemental analysis and optical measurements. The binding properties of the five different OPV guests to a N,N-bis[(3-adamantyl ureido) propyl] methylamine host have been investigated. The guests that contained an aryl urea glycine spacer were bound with the highest association constant. Subsequently, an adamantyl urea modified fifth generation poly(propylene imine) dendrimer was synthesized as a multivalent host which contains 32 N,N-bis[(3-adamantyl ureido) propyl] amine binding sites. Size exclusion chromatography showed that 32 of the OPV guests strongly bind to the fifth generation adamantyl functionalized dendritic host. In the case of the supramolecular dendritic host/guest system smooth homogeneous thin films could be obtained by spin coating. The dendritic guest-host complexes showed a significantly higher emission upon binding then that of the individual molecules due to the three-dimensional orientation of the OPV guest molecules. In the solid state, this enhancement in luminescence was a factor of 10. The pi-conjugated oligomers are less aggregated in the supramolecular assemblies presumably because of a shielding effect of the bulky adamantyl units present in the hosts.

Supramolecular organization of alpha,alpha'-disubstituted sexithiophenes.

The self-assembly of alpha,alpha'-linked sexithiophenes with chiral and achiral penta(ethylene glycol) chains attached at the alpha-positions of the terminal rings, that is, 2,2':5',2'':5'',2''':5''',2'''':5'''',2'''''-sexithiophene-5,5'''''-dicarboxylic acid-2S)-2-methyl-3,6,9,12,15-pentaoxahexadecyl ester (1) and 2,2':5',2'':5'',2''':5'''',2''''':5''''',2'''''-sexithiophene-5,5'''''-dicarboxylic acid-3,6,9,12,15-pentaoxahexadecyl ester (2), respectively is described. Analysis of the UV/vis, fluorescence, circular dichroism, and circular polarization of luminescence spectroscopic data shows that these compounds form chiral aggregates in polar solvents and in the solid state. In n-butanol aggregation occurs at temperatures below 30 degrees C, while above this threshold temperature the aggregates break up without an intermediate disordered state of aggregation, and the compounds are molecularly dissolved. The "melting temperature" of the aggregates depends on the concentration of sexithiophene, indicating that the optical changes observed are a result of intermolecular processes. Mass spectrometric measurements reveal that 1 and 2 can form mixed aggregates. Analysis of the optical spectra reveals that in these mixed aggregates, chiral 1 molecules act as "sergeants" to direct the packing of the "soldiers" 2, illustrating cooperativity within the columns. In water, the same type of chiral aggregates are formed as in n-butanol below 30 degrees C; however, these aggregates are still present, but the chirality is lost above 30 degrees C. In spin-coated films of 1 chiral aggregates are present. AFM studies show that 1 self-organizes into chiral fiberlike structures in the solid state. Furthermore both 1 and 2 display thermotropic liquid crystalline behavior between 180 and 200 degrees C.